The invention relates to an apparatus and a process for the combined purification and compression of CO-containing hydrogen and to the use of the hydrogen thus obtained as a fuel gas in polymer electrolyte membrane (PEM) fuel cells.
Fuel cells are distinguished by high electric efficiency, low pollutant emissions and low noise levels and are therefore highly suitable to drive vehicles. Finding favour for this application is the polymer electrolyte membrane (PEM) fuel cell, given its operating temperature range, its robustness and its tolerance with respect to pressure fluctuations. PEM fuel cells do, however, have to be operated with pure hydrogen (H2) as a fuel. On the other hand, it is advantageous that, as an alternative to oxygen (O2), air can be used as the oxidant. CO fractions above 20 ppm in the hydrogen used as a fuel gas cause poisoning of the electrode and a marked drop in performance in conjunction with excessive specific energy consumption.
The purification of hydrogen conventionally takes place discontinuously by means of pressure swing adsorption (PSA). The processes based on this technique do however involve high specific energy consumption, require a bulky construction and are therefore unsuitable for operating vehicles.
Known mechanical processes for compressing hydrogen comprise, for example, the use of piston compressors, screw compressors and rotary compressors. Such methods do, however, involve high noise levels and, particularly in the case of installations carrying an electric load of up to 10 kW, require a high specific energy input.
GB-A-2 268 322 discloses that pure hydrogen can be compressed electrochemically, using PEM cells, to up to a differential pressure of 5 bar. Above such a differential pressure the electrode mountings of the known PEM cells are subject to cracking.
Hydrogen produced as a reformer gas from methanol or other hydrocarbons contains varying amounts of carbon monoxide (CO). The provision of ultrapure, CO-free hydrogen from such reformer gases, in particular for the cold-start phase of fuel cell vehicles, while at the same time ensuring a load change behaviour on a timescale ranging from milliseconds to seconds, is technically not feasible as yet.
The object of the present invention is therefore to provide a low-noise and energy-efficient apparatus and a process for the combined purification and compression of CO-containing hydrogen, differential pressures of at least about 10 bar being achievable.
This object is achieved according to the invention by an apparatus according to Claim 1 and a process according to Claim 9. Advantageous or particularly expedient refinements of the subject-matter of the invention are specified in the subordinate claims.
The invention therefore relates to an apparatus for the combined purification and compression of CO-containing hydrogen, comprising a polymer-electrolyte membrane-(PEM)-cell including a proton-conducting polymer membrane, an anode on one side and a cathode on the other side of the membrane, the anode being in the form of a bilayer anode which comprises a CO-oxidation-selective catalyst layer on the side facing away from the membrane and an electrochemically active layer for oxidizing the hydrogen to protons on the side facing the membrane, and further including a planar, porous support layer which is provided at the anode side and adjoins a current collector, so that the PEM cell is able to withstand differential pressures of at least about 10 bar.
The invention further relates to a process for the combined purification and compression of CO-containing hydrogen by means of an apparatus according to the invention, wherein the CO in the CO-containing hydrogen is oxidized selectively to CO2 in the first PEM cell, by means of the hydrogen being admixed with an amount of oxygen corresponding to the CO content, said oxygen catalytically oxidizing the CO on the CO-oxidation-selective catalyst layer of the bilayer anode to give CO2, and wherein the now largely CO-free hydrogen is compressed electrochemically by the PEM cell to differential pressures of at least about 10 bar, by virtue of the electrochemically active layer of the bilayer anode oxidizing the hydrogen to protons which, having passed the membrane at the cathode side, are discharged under pressure to give ultrapure hydrogen.
Finally, the invention relates to the use of a hydrogen obtained in accordance with the process according to the invention as a fuel gas in PEM fuel cells, especially for vehicles.